1. Field of the Invention
The present invention relates to an optical scanning apparatus that includes a function for controlling dot positions in a sub-scanning direction, a control method of the optical scanning apparatus, and an image forming apparatus.
2. Description of the Related Art
In general, an image forming apparatus forms a latent image by deflecting a light beam using a polygonal mirror and repeatedly scanning the surface of a photosensitive drum with that light beam. Such scanning shall be referred to as “main scanning” hereinafter. Furthermore, the linear-shaped latent image formed by this main scanning shall be referred to as a “main scanning line”.
An image forming apparatus typically performs a single main scan for a single surface of the polygonal mirror, thereby forming a single main scanning line. For example, when a polygonal mirror having four reflective surfaces is used, and that polygonal mirror rotates once, four main scans are performed, thereby forming four main scanning lines. It is of course preferable for the four main scanning lines to be formed at equal intervals, but in reality these intervals are not equal. This is called “unevenness of pitch” in the sub-scanning direction.
A conventional invention for reducing unevenness of pitch of the main scanning lines caused by unevenness of rotation of the photosensitive drum, unevenness of rotation of a transfer belt, and so on, has been proposed (Japanese Patent Laid-Open No. 2004-37757). According to this invention, unevenness of pitch is corrected by disposing a micromirror, which is capable of minute resonance at a slower rate than the main scanning frequency, within the optical path. Such a technique allows useful correction effects to be obtained with respect to “unevenness of pitch caused by low-frequency components” resulting from “sinusoidal undulations” in the sub-scanning direction.
FIG. 14 is a diagram illustrating this “unevenness of pitch caused by low-frequency components” resulting from “sinusoidal undulations”, which is caused by unevenness of pitch of the main scanning lines caused by unevenness of rotation of the photosensitive drum, unevenness of rotation of a transfer belt, and so on.
However, there are other factors that lead to unevenness of pitch. While it is preferable to prepare the polygonal mirror so that its reflective surfaces each have equal inclination angles relative to the rotational axis, this is, in reality, difficult to accomplish, in terms of the manufacturing accuracy. Slight differences thus arise in the inclination angles of the reflective surfaces. Such differences in the inclination angles shall be referred to as “surface inclination” hereinafter. This surface inclination leads to ununiformity in the reflection angles of the light beam, and unevenness of pitch in the main scanning lines occurs as a result.
The unevenness of pitch in the main scanning lines caused by the surface inclination occurs at the same speed as the main scanning frequency (several kHz), and thus does not occur slowly compared to the main scanning frequency. Accordingly, in the present specification, this shall be called “unevenness of pitch caused by high-frequency components” rather than “unevenness of pitch caused by low-frequency components”.
FIG. 15 is a diagram illustrating an example of unevenness of pitch in the main scanning lines caused by surface inclination (that is, unevenness of pitch caused by high-frequency components). While the unevenness of pitch illustrated in FIG. 14 has a frequency of approximately several Hz to several tens of Hz, the unevenness of pitch illustrated in FIG. 15 has a frequency of several kHz, and thus the two are clearly substantially different from one another.
Meanwhile, the unevenness of pitch in the main scanning lines caused by surface inclination in the polygonal mirror has a square-wave shape. Therefore, such unevenness of pitch is not caused by the “sinusoidal undulations”, such as unevenness of pitch of the main scanning lines caused by unevenness of rotation of the photosensitive drum, unevenness of rotation of a transfer belt, and so on (see FIG. 15). Accordingly, because the sub-scanning unevenness of pitch is square wave in form, a resonant driving type micromirror cannot sufficiently correct the unevenness of pitch when being driven by a sine wave.
However, if the resonant driving type micromirror can be driven by a square wave, it seems possible to reduce the unevenness of pitch caused by the surface inclination. However, as illustrated in FIG. 16, an oscillation phenomenon (indicated by the ovals in FIG. 16) occurs when the resonant driving type micromirror is simply driven by a square wave. It takes approximately 100 ms for this oscillation phenomenon to subside, making it extremely difficult to effectively correct unevenness of pitch occurring in the main scanning cycle, which is several hundred μs.